Flat rolled ferrous metal for vitreous enameling



Patented May 4, 1954 FLAT ROLLED FERROUS .METAL FOR "VITREOUS ENAMELINGJoseph C. 'Eckel, Pittsburgh, Pa., assignor "to United States SteelCorporation, a corporation of New Jersey N Drawing..ApplicationFebi-uary 15, 1951,

Serial No. 211,187

4 Claims. 1

This invention relates to the preparation of flat rolled ferrous metalfor vitreous enameling and, particularly, to the preparation of flatrolled ferrous metal for vitreous enameling which is free from surfacedefects caused by gas evolution therefrom during the enameling process.

One of the oldest and most vexing problems incident to the manufactureof enamelware formed by fiat rolled ferrous metal coated with vitreousenamel is an unsightly surface condition caused by the presence of blackor brown spots or specking. It results either in rejection of the wareor necessitates expensive reoperations, such as stripping, spraying andfiring, after which the Ware may still be rejected.

Those skilled in the art have for years directed their attention tosolving the problem incident to such surface defects. While at timesfully satisfactory enamelware has been produced, it Was never possibleto assure uniformly satisfactory" results. It is common for thesesurface defects to occur in the enameling of one lot of sheets whileanother lot made to exactly the same specifications is free from them.At times, certain sheets of a lot may show the defect, andothers fromthe same lot do not exhibit any traces of it. In other words, there hasbeen an uncontrollable and unaccountable lack .of uniformity in results,and the best efforts of those most highly skilled in the art haveheretofore failed to provide a complete solution to the problem.

I have discovered that such surface defects can be eliminated byagglomerating carbides present in the metal into concrete masses of thelargest possible dimensions and then dispersing theseagglomeratedcarbides, with simultaneous degasification of the metal, prior to theenameling.

I agglomerat the carbides in the metal by critically straining and hightemperature box annealing, and I disperse the agglomerated carhides anddegasify the metal prior to enameling by a normalizing treatment, whichI shall explain,

The box annealing treatment of the flat rolled ferrou material iseffected by heating the stock in th range of 1225 to 1400 FL, preferablyin a narrower range of substantially 1225 to 1315 F. The metal ismaintained at temperature during the annealing treatment for at leastabout six hours, best results ordinarily being obtained by keeping themetal within this temperature range for a time approaching twenty hours.

Heatin in the temperature range required for the agglomeration ofcarbides is preferably followed by slow cooling from this temperaturerange to a temperature at least as low asabout preferably in thenarrower range of to F. per hour.

Before box annealing, I impart to the hot rolled stock an amount of coldreduction which is regulated to critically strain the material. Whilethis amount of cold reduction may vary somewhat, dependent, oncomposition and prior treatment, it is less than 15% and ordinarily inthe range of 5 to 8%. In certain cases the metal may be cold reducedboth before and after agglomeration of carbides. For example, before theagg1omeration of carbides, the metal may be cold reduced in thepreferred narrow range of 5 to 8%, and after box annealin the metal mayb further cold reduced to a greater extent. The total cold reductioneither in one or two rollings may, in some cases, be as much as Thedispersion of agglomerated carbides and degasifying of metal ispreferably accomplished by heating the metal in the range ofsubstantially *1600 to 1850 F., preferably about 1750 F., metaltemperature, and cooling it in air.

While my invention may be practiced using any flat rolled ferrous metalsuitable for vitreous "enameling, I obtain superior results when the-carbon and manganese contents are maintained desirably-low. Forexample, a ferrous metal of the following typical analysis:

Per cent Carbon .03 Manganese .07 Phosphorus .015 Sulphur .05 f'Silico'n.015

when treated as herein described produces excellent results.

In reducing my invention to practice, I hot roll blooms of enamelingiron to substantially 0.080 age, coil the strip hot, cold work reducethe coiled strip to critically strain it, box anneal the criticallystrained strip at 1225 to 1315" FL, using a 12-hour soak, and normalizeannealed stock at 1725 to 1775 F., preferably in a neutral or a reducingatmosphere.

The process involved, while not perfectly understood at present, dependson gas evolution control. Enameling irons always contain some oxygen.During heating at the enameling temperature, this oxygen combines withcarbon present in the metal, and the resulting gas escapes through theviscous layer of fused enamel in the shap of bubbles which collapse onreaching enamel surface, frequently extending from the metal to thesurface of the enamel. Bursting of the bubbles exposes the metallic baseto the oxidizing effect of furnace atmosphere, leading to the formationof the above-mentioned surface defects.

The present invention avoids gas generation in firing by eliminatingoxygen from the carboncontaining base prior to enamel firing operation.

Carbon present in solid solution in alpha iron is first coagulated intolarge masses of cementite b annealing under the A1 point. Since cementitcoagulates at the grain boundaries and the size of its masses is afunction of grain size, the application of critical strain byappropriate amount of cold work disclosed above greatly improves theefiectiveness of the process. Heating coagulated cementite above thecritical point transforms it into an austenitic grain having a eutectoidconcentration of carbon assuring in this grain an instantaneousconsumption of oxygen, which escapes as CO in. the atmosphere. At thetime of the enamel-firing operation, there is no oxygen remaining tocome in contact with carbon and cause the surface defects.

When flat rolled ferrous metal is prepared for vitreous enameling by myprocess, uniformly satisfactory results are assured virtually 00% of thetime. Flat rolled ferrous metal prepared by m process can be enameled inconventional manner by applying the ordinary blue undercoat and white orlight colored finish coat, or by employing a single coat of white orlight colored enamel. In either case, reoperations due to the surfacedefects are virtually eliminated.

This application is a continuation-impart of my copending applicationSerial No. 710,948, filed November 19, 1946, now abandoned.

I claim:

1. The method of eliminating oxygen from lowcarbon undeoxidized steelenameling stock and thereby prevent reboiling during firing, comprisingimparting a critical strain thereto by cold rolling to insure maximumgrain growth when annealed, annealing said stock at a temperature belowits A1 point to cause grain growth therein and to coagulate the carbonin said stock into large masses of cementite along the large grainioundaries and then normalizing said stock to transform said cementiteinto austenite grains having a eutectoid carbon concentration thatcombines with oxygen, said combined oxygen being then dispersed from thestock as carbon monoxide.

The method of eliminating oxygen from lowcarbon undeoxidized steelenameling stock and thereby prevent reboiling during firing, comprisingimparting a critical strain thereto by cold 4 reducing it between 5 and8% to insure maximum grain growth when annealed, annealing said stock ata temperature below its A1 and between 1225 and 1400" F. point to causegrain growth therein and to coagulate the carbon in said stock intolarge masses of cementite along the large grain boundaries and thennormalizing said stock at a temperature between 1600 and 1850 F. totransform said cementite into austenite grains having a eutectoid carbonconcentration that combines with oxygen, said combined oxygen being thendispersed from the stock.

3. The method of preventing reboiling of enameling stock during firingof enamel coatings thereon, comprising forming enameling stock oflowcarbon undeoxidized steel, imparting a critical strain thereto bycold rolling to insure maximum grain growth when annealed, annealingsaid stock at a temperature below its A1 point to cause grain growththerein and to coagulate the carbon in said stock into large masses ofcementite alon the large grain boundaries, normalizing said stock totransform said cementite into austenite grains having a eutectoid carbonconcentration that combines with oxygen, said combined oxygen being thendispersed from the stock as'carbon monoxide to prevent reboiling duringfiring, placing a coat of enamel frit thereon and then firing it.

4. The method of preventing reboiling of enameling stock during firingof enamel coatings thereon, comprising forming enameling stock oflow-carbon undeoxidized steel, imparting a critical strain thereto bycold reducing it between 5 and 8% to insure maximum grain growth whenannealed, annealing said stock at a temperature below its A1 and between1225 and 1400" F. point to cause grain growth therein and to coagulatethe carbon in said stock into large masses of cementite along the largegrain boundaries, normalizing said stock at a temperature between 1602)and 1850" F. to transform said cementite into austenite grains having aeutectoid carbon concentration that combines with oxygen, said combinedoxygen eing then dispersed from the stock to prevent reboiling duringfiring, placing a coat of enamel frit thereon and then firing it.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date ,581,269 Kelley Apr. 20, 1926 1,996,568 Butts Apr, 2,1935 2,065,392 Porter Dec. 22, 1936 2,303,343 Engel Dec. 1, 19422,455,331 Eckel et a1 NOV. 30, 1948 2,532,640 Pfeiffer Dec. 5, 1950

4. THE METHOD OF PREVENTING REBOILING OF ENAMELING STOCK DURING FIRINGOF ENAMEL COATINGS THEREON, COMPRISING FORMING ENAMELING STOCK OFLOW-CARBON UNDEOXIDIZED STEEL, IMPARTING A CRITICAL STRAIN THERETO BYCOLD REDUCING IT BETWEEN 5 AND 8% TO INSURE MAXIMUM GRAIN GROWTH WHENANNEALED, ANNEALING SAID STOCK AT A TEMPERATURE BELOW ITS A1 AND BETWEEN1225 AND 1400* F. POINT TO CAUSE GRAIN GROWTH THEREIN AND TO COAGULATETHE CARBON IN SAID STOCK INTO LARGE MASSES OF CEMENTITE ALONG THE LARGEGRAIN BOUNDARIES, NORMALIZING SAID STOCK AT A TEMPERATURE BETWEEN 1600AND 1850* F. TO TRANSFORM SAID CEMENTITE INTO AUSTENITE GRAINS HAVING AEUTECTOID CARBON CONCENTRATION THAT COMBINES WITH OXYGEN, SAID COMBINEDOXYGEN BEING THEN DISPERSED FROM THE STOCK TO PREVENT REBOILING DURINGFIRING, PLACING A COAT OF ENAMEL FRIT THEREON AND THEN FIRING IT.